The present invention is related to temperature measurement, and more particularly to temperature measurements using a transistor or diode as a sensor.
Temperature measurement using a transistor as a sensor is a common application in the semiconductor area. Such a temperature measurement is done by applying two different currents to the transistor resulting in two different base-emitter voltages. The difference between the two base-emitter voltages is proportional the absolute temperature of the transistor. Turning to FIG. 1a, an example of such a temperature measurement circuit 100 is shown. Temperature measurement circuit 100 includes a transistor 120 that is used as a temperature sensor. The collector and the base of transistor 120 are electrically coupled to a variable current source 110. Further, the base of transistor 120 is electrically coupled to one input of an analog to digital converter 130, and the emitter of transistor 120 is electrically coupled to another input of analog to digital converter 130. Analog to digital converter 130 is operable to receive the voltages at the base and emitter of transistor 120, and to provide a ΔVbe output 135 representing the difference between two different base to emitter voltages. ΔVbe output 135 is provided to a temperature calculation circuit 140 that provides an uncorrected temperature output 145.
As previously mentioned, the difference between the two base-emitter voltages is proportional to the absolute temperature of transistor 120. The following equation defines the relationship between the difference between base-emitter voltage measurements and absolute temperature:ΔVbe=Vbe2−Vbe1=n*kT/q*ln(I2/I1).The ‘n’ term is known as the non-ideality factor or emission coefficient is assumed to be a constant (n=1.008) for diodes and transistors. Most sensors expect the n-factor to have a constant value of 1.008. In reality, the non-ideality factor is a function of the structure and fabrication process of transistor 120 and as semiconductor processes move to smaller geometry transistor 120 becomes something other than the expected value of 1.008. The following equation represents the temperature error seen if the non-ideality factor is different that the presumed 1.008:Terror=((n−1.008)/1.008)*273.15*Tideal.
To correct for the aforementioned temperature error, some circuits have included a backend offset circuit designed to add or subtract a calculated constant from uncorrected temperature 145 and thereby achieve a corrected temperature. FIG. 1b shows an example of one such temperature calculation circuit 101. As shown, temperature calculation circuit 101 is substantially similar to temperature calculation circuit 100, except for the addition of a temperature offset adder circuit 150. Temperature offset adder circuit 150 receives uncorrected temperature 145 and a programmed temperature offset input 147. The two inputs are added together to create a corrected temperature output 155. While such an offset approach can effectively correct calculation errors at a given point on an operational curve, the inaccuracy of the calculated temperature still exists as operation moves farther from the aforementioned offset corrected point on the operational curve.
Thus, for at least the aforementioned reasons, there exists a need in the art for advanced systems and devices for temperature measurement.